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V751 Cyg: A Neglected VY SCL star

Gary Poyner

Of all the Nova like stars, the VY Scl type subgroup are perhaps one of the most interesting. Commonly (and misleadingly) known as 'Anti Dwarf Novae' they are usually found varying in the order of one magnitude at maximum brightness. At random intervals deep fades occur, which can last from days to weeks or even years. This led to the VY Scl stars being classified erroneously as type RCB until a few years ago. An examination of the spectra during low states however, reveals a remarkable similarity to Dwarf Novae in quiescence.

On rare occasions during low states, dwarf novae type outburst occur. These are usually short duration events (a few days at most), and are by no means common in all VY Scl type objects. Probably the most famous star to exhibit this phenomenon is MV Lyr.

V751 Cygni is a VY Scl star with a range of 4.5 magnitudes (13.5-18.0V), and is associated with the Cygnus T1 association (IC5070). I first became aware of V751 Cygni when it was included in a ROSAT CV target list in 1990, which I was contributing observations to. The light curve from my own visual observations included here shows that V751 Cygni varied around one magnitude from 1990 until March 1997, when the Hungarian observer, Laslo Szentasko recorded a fade to magnitude 15.6. The fade continued quickly, until a deep V magnitude measurement of 17.8 was obtained by Ouda observatory, Kyoto, Japan during April 1997. This proved to be the first ever deep low state of V751 Cygni to be obsered visually and with CCDs. Following this, V751 Cygni slowly began to recover until it had reached magnitude 16 by the end of that year. During 1998, the slow recovery was maintained, until the star is now approaching what might be termed its 'normal' high state.

A recent paper by Jochen Greiner1, reveals that ROSAT observations during the recent low state, found V751 Cygni to be a highly luminous source of soft X-rays - possibly indicating that all VY Scl stars sra SSBs (Supersoft Binaries).

Unfortunately V751 Cygni has not attracted too many visual observers in the past, and presently there are just two or three people around the world monitoring it on a regular basis. Considering how little we know of its optical behaviour, and that we have just monitored the first optical low state to be seen, V751 Cygni surely deserves more attention from visual and CCD observers alike. To this end, V751 Cygni will be added to the telescopic programme in the hope that more observers will add it to their observing programmes - especially CCD users!


1 Relation between Supersoft X-ray sources and VY Scl stars - J. Greiner et al. www download from

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